NASA and SpaceX are again investigating the parachutes that help land the spacecraft the company uses to fly cargo and crew to and from the International Space Station, after one of the parachutes opened slowly after a recent mission.

In November, one of the four parachutes on a Dragon spacecraft carrying a crew back to Earth took longer to inflate than the other three. The issue happened again last month, NASA said Friday, with a version of the spacecraft designed to ferry cargo and supplies to the station. In each case, one parachutes lagged by about a minute.

The issue poses no danger to the astronauts who are now on the space station and scheduled to return in April, NASA and SpaceX said. The system is also designed to land safely if only three of the parachutes deploy. But they said they want to do everything they can to ensure that the system is as safe and robust as possible.

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“We’re going to go back and double check to see if we can understand some more of the finer points of how these ships operate and work,” said Bill Gerstenmaier, SpaceX’s vice president for build and flight reliability. “But it’s not a safety concern. It’s not an issue for the crew. There’s no modifications required on orbit. This is more of a learning exercise of how we can improve our design and engineering understanding of parachute operation.”

The investigation was first reported by SpaceNews.

Gerstenmaier said that if you didn’t see the chute take a moment to inflate after the others, and were only looking at the descent data, you wouldn’t know it lagged behind the others.

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“So the system is operating correctly,” he said. “But even though the system is operating correctly, we’re not happy with that. We’re going to keep looking and see if we can understand better how the system actually operates, and understand where the weaknesses are in the system so we can have a safer system for crews moving forward.”

After the Crew-2 return, officials said they thoroughly inspected the chutes and found no problem. And they said that the descent rate of the capsule was well within the safety margins. When the cargo capsule returned last month, the rate of descent was also fine, officials said.

NASA and SpaceX believe the lag on the fourth parachute occurs because the others essentially crowd it out, “and then it struggles to inflate at times,” said Steve Stich, NASA’s commercial crew program manager.

He said it had happened on other cargo missions as well, but couldn’t say how many.

“It is interesting to us that we saw it on back-to-back missions,” he said. “So we’re taking the extra time to continue to look at the system.”

SpaceX’s next human spaceflight mission, Crew-4, is slated for mid-April and the return of Crew-3 is scheduled for the end of that month.